Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Tue Aug 20 2019 Valid 12Z Fri Aug 23 2019 - 12Z Tue Aug 27 2019 ...Overview and Guidance/Predictability Assessment... 16Z Update: Minor changes needed from previous forecast issuance for this cycle. The main notable model guidance change toward the latter portions of the forecast period where the 00Z ECMWF trended toward ridging off the New England coast (12Z run yesterday showed a closed low forming Sunday), which is more aligned wit the rest of the deterministic and ensemble guidance. Thus, more inclusion of the ECMWF and its ensemble mean was included for Day 5-7. Otherwise, fairly typical summertime pattern expected across the CONUS with a couple shortwaves tracking across the northern tier of the U.S. ---previous discussion--- An upper trough extending from eastern Canada into the northeastern U.S. as of early Fri will progress eastward with time while fast flow arriving from the North Pacific should develop broad mean troughing across the northern tier states from the weekend into early next week. Upper level ridges should prevail over the southern Rockies/Four Corners region and over the eastern Pacific (the latter ridge possibly extending into the western U.S.). Guidance suggests a relative weakness aloft will persist over the western Gulf Coast/Lower Mississippi Valley. The most prominent uncertainties involve specifics of the northeastern trough and combined details of northern tier flow/upstream ridge. Among the 12-18Z guidance one forecast consideration was to downplay some 12Z ECMWF details, as it and its mean were more amplified than other solutions for the shortwave reaching the northern High Plains by Fri while the operational run developed a closed low just off the New England coast by Sun. The previous ECMWF run also formed a closed low near New England. This scenario was within the full ensemble spread but so far confidence was too low to depict in a single deterministic forecast. Interestingly the new 00Z UKMET/CMC use farther west shortwave energy to close (CMC) or nearly close (UKMET) an upper low that crosses New England on Sat with faster timing than the ECMWF by Sun. At this time the main idea is that it could take some additional time for troughing to cross New England after the core of the initial upper trough crosses the Canadian Maritimes. Returning to the northern Plains energy, new 00Z runs still suggest the 12Z ECMWF/ECMWF mean could be on the strong side. By the latter half of the period guidance is still showing considerable sensitivity regarding eastern Pacific/western U.S. upper ridge details and effects on northern tier troughing. Through the 12-18Z cycles the ECMWF/CMC and their means have been fairly consistent (minus a modest slower trend) for at least a couple days in showing a strong enough eastern Pacific into western U.S. ridge to yield a downstream trough axis over the northern Plains/Upper Midwest by next Mon-Tue. Recent GEFS means have been tending to show a farther westward ridge/trough while GFS runs have varied between the two camps. The 18Z GEFS mean had elements of both. The positive height anomaly center seen in multi-day means late in the period have generally been favoring the ECMWF/CMC cluster but latest means show potential for the anomaly center to be just a little farther west--and according to teleconnections possibly enough to change the character of the trough. So preference leaned about two-thirds toward the ECMWF/ECMWF mean cluster relative to the 18Z GEFS mean but with somewhat lesser confidence than was previously the case. New 00Z CMC/ECMWF runs that have adjusted westward further lower confidence in a specific solution. ...Weather Highlights/Threats... Compared to yesterday the latest guidance is showing a stronger signal for heavy rain potential over parts of the northern Plains/Upper Midwest and possibly into the Great Lakes. As strong Pacific flow carves out a mean trough over the northern tier states, leading height falls aloft and associated cold front(s) plus a leading warm front may progress along slowly enough to allow for some training/repeating convection. However it will take time to resolve important lower predictability shortwave details and significant changes in the forecast are possible. Another area of interest for possible heavy rainfall will extend from the west-central Gulf Coast through the Southeast/southern Mid-Atlantic. A wavy front settling into the Southeast will provide a multi-day focus for rainfall while energy aloft/moisture within or east of the general upper weakness over the western Gulf Coast/Lower Mississippi Valley may help to generate one or more episodes of enhanced rainfall near the Gulf Coast. Some of this moisture could end up interacting with the Southeast front. The primary area of above normal temperatures should extend from southern Oregon and northern-central California into the southern High Plains with some pockets of plus 10F or greater anomalies for highs and/or morning lows. The northern High Plains may see highs at least 5-10F below normal this Fri and then again early next week. Forecast confidence over parts of the West and Plains decreases considerably by next Mon-Tue though. Southern Canada high pressure building into New England and clouds/rain with the front to its south should provide near to below normal highs over the eastern U.S. for multiple days. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml